The present invention relates to a cooling system in a vehicle including an arrangement for controlling the circulation of fluid to different elements of the system.
Heavy vehicles are often equipped with one or several supplementary brakes in order to reduce wear on the ordinary wheel brakes of the vehicle. Such a supplementary brake may be a hydraulic retarder. Oil is used as the working medium in one type of hydraulic retarder. When the vehicle is braked with the aid of the retarder, the oil undergoes rapid heating. The hot oil is led to a heat exchanger where it is cooled by cooling fluid that circulates in the cooling system of the vehicle. The cooled oil is subsequently returned to the retarder for further use. In another type of hydraulic retarder, cooling fluid is used instead of oil as working medium in a retarder. In this case, the cooling fluid receives direct heating in the retarder.
The cooling system generally comprises a circuit in which the cooling fluid first cools the combustion engine and subsequently the retarder. In conditions in which the retarder is not activated, it does not generate any heat energy and thus does not need to be cooled. In such conditions, the cooling fluid receives instead a cooling effect when it is led through the heat exchanger or retarder. The cooling of the cooling fluid as it is led through the retarder or heat exchanger extends the period it takes for it to reach a desired operating temperature after a cold start. This cooling also makes it more difficult for the cooling fluid to be heated to a desired operating temperature on occasions when the vehicle is driven in cold surroundings. The continuous flow of cooling fluid through the retarder or heat exchanger results also in an extra fall in pressure in the cooling system.
WO 98/15726 reveals a cooling system in a vehicle with a circulating cooling fluid that cools a combustion engine and a retarder. On occasions when the retarder is not activated, cooling fluid can be led past the retarder through a bypass line. The cooling system comprises a conventional thermostat that leads cooling fluid to a cooler when it has a temperature that is higher than a regulation temperature, and to a return line without cooling when it has a temperature that is lower than the regulation temperature. The cooling fluid is circulated in the cooling system with a flow that can be varied, with the aid of one or several cooling fluid pumps. A control unit controls the flow of cooling fluid in the cooling system with the aid of the cooling fluid pump and possibly also a control valve, such that the temperature of the combustion engine does not exceed a pre-determined maximum temperature.